U.S. Border Patrol agent Sal De Leon stands near a section of the U.S.- Mexico border fence while stopping on patrol on April 10, 2013 in La Joya, Texas. According to the Border Patrol, undocumented immigrant crossings have increased more than 50 percent in Texas' Rio Grande Valley sector in the last year.

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A U.S. Border Patrol agent detains undocumented immigrants who had crossed from Mexico into the United States on April 11, 2013 in Mission, Texas. In the last month the Border Patrol's Rio Grande Valley sector has seen a spike in the number of immigrants crossing the river from Mexico into Texas. With more apprehensions, they have struggled to deal with overcrowding while undocumented immigrants are processed for deportation.

By Mark Potter , Correspondent, NBC News

In the small town of San Juan, Texas, a few miles north of the Mexican border, Police Chief Juan Gonzalez toured some of the human stash houses his officers recently uncovered. They had been used to hide immigrants from all over the world who were smuggled across the border into the United States.

Gonzalez says his department has never dealt with as many undocumented immigrants as it encounters now.

"In the past three years we've seen an increase. And it's not a steady increase, it's a massive increase," he said. "Too many people are getting through. We've got too many holes in the border and we don't have enough manpower to make sure we secure the border."

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U.S. Office of Air and Marine agent Jake Dreher stands over a drug smuggler on the bank of the Rio Grande River at the U.S.- Mexico Border on April 11, 2013 in Mission, Texas. Agents with helicopter support from the U.S. Office of Air and Marine broke up a marijuana smuggling operation from Mexico into Texas.

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A U.S. Border Patrol canine team works with an U.S. Air and Marine agent to detain an undocumented immigrant after chasing him down near the U.S.- Mexico border on April 11, 2013 near Mission, Texas. A group of 16 immigrants from Mexico and El Salvador said they crossed the Rio Grande River from Mexico into Texas during the morning hours before they were caught.

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A U.S. Border Patrol agent looks for drug smugglers on the bank of the Rio Grande River at the U.S.- Mexico Border on April 11, 2013 in Mission, Texas.

About 75 miles north of the border, in Falfurrias, Texas, Benny Martinez, the chief deputy of the Brooks County Sheriff's Office, says his area is also deeply affected by a recent rise in illegal immigration.

“The trending is going up,” he said. “It hasn’t gone down at all, not here.”

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Undocumented immigrants from El Salvador sit handcuffed after being detained by the U.S. Border Patrol near the U.S.- Mexico border on April 11, 2013 near Mission, Texas.

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Suspected drug smugglers flee across the Rio Grande River into Mexico on April 11, 2013 in Mission, Texas. Their marijuana smuggling mission was broken up by U.S. Border Patrol agents with helicopter support from the Office of Air and Marine.

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A Honduran mother holds her toddler son at the U.S. Border Patrol detainee processing center on April 11, 2013 in McAllen, Texas. They had been caught by the Border Patrol while crossing illegally from Mexico into Texas.

Last year, officials and ranchers there found the bodies of 129 immigrants who died in the harsh terrain, presumably after crossing the border illegally. Dozens are still unidentified and are buried in a local cemetery. Some of the metal markers on the graves read, "Unknown Female" and "Unknown Remains." One says, simply, "Bones."

Martinez does not believe the U.S.-Mexican border is at all secure in South Texas, given the rise in illegal immigration in Brooks County.

"It's steady and I don't think it's going to go down, it's not going to happen anytime soon," he said.

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A U.S. Border Patrol Agent inspects a load of marijuana seized from drug smugglers near the U.S.- Mexico border on April 10, 2013 in Hidalgo, Texas.

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A U.S. Border Patrol agent searches in dense brush for undocumented immigrants who had crossed from Mexico into the United States on April 11, 2013 in Penitas, Texas.

Despite a dramatic drop in illegal immigration nationwide, south Texas, along the Rio Grande, is now seeing a rise in immigrants crossing the Mexican border, as many flee the poverty and violence in Central America. NBC's Mark Potter reports.